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In the world of spacecraft, every gram costs money,
so if you want the cheapest ship, you’ve got to build the smallest ship.
That’s the promise of CubeSats: tiny satellites at bargain-basement prices.
The smallest measure just ten centimeters on a side, roughly the size of a Rubik’s cube,
and weigh a little more than a kilogram.
Since the first launch in 1998, they’ve become a key tool
for universities and companies looking to carry out small, focused experiments.
CubeSats are incredibly versatile,
but it’s a big challenge to pack everything you need into such a tiny space.
After all, you still need the instruments for your CubeSat mission,
plus a power source, computer, communication system, and a way to get around.
Not all of those features always make the final cut.
In fact, almost every CubeSat launched so far has had little or no propulsion,
leaving it without much control over where it goes.
To expand the applications of these nanosatellites, they need a way to move,
and that’s driving the development of new, miniaturized propulsion systems.
We’ve looked at a couple of these before, like electrospray and pulsed plasma thrusters.
But these are so cool, let’s check out a couple more.
One option is cold gas propulsion.
This is pretty much exactly what it sounds like.
The CubeSat stores a gas like butane, argon, or nitrogen under pressure and releases it to move.
The gas vents out into space, producing thrust in the other direction
that pushes the satellite along, just like a fire extinguisher on an office chair!
This kind of mechanism has already been tested in space,
including on a tiny satellite launched from Space Shuttle mission STS-116 back in 2006.
Right now, engineers are looking at ways of 3D printing the entire system,
to make it as small, lightweight, and sturdy as possible.
Cold gas thrusters have the advantage of being incredibly simple,
needing only valves to control the flow of gas.
Plus, the gases used are unreactive on their own, meaning there’s
relatively little risk to the rest of the structure, or the rocket, if anything goes wrong.
Although a flammable gas like butane is more hazardous than an inert one like argon.
Because of this simplicity, the cold gas system can also produce thrust for relatively little weight,
allowing the propulsion needed for a mission to take up a smaller fraction of the total payload mass.
But, as its name suggests, cold gas is cold, so each gram of cold propellant
will provide comparably less thrust than an equivalent amount of hot exhaust gas.
This means the system has a low specific impulse, which is kind of like the fuel efficiency of a car.
Cold gas propulsion will certainly get your cubesat moving, but it won’t get you particularly far.
For that reason, this approach is often considered just for attitude control,
like getting pointed the right way, rather than as the main means of getting around.
Now, if cold gas thrusters are one of the simplest propulsion methods,
then electron-cyclotron resonance, or ECR, is one of the most complicated.
This method uses microwaves to generate plasma for thrust.
Gas starts in the chamber at low pressure.
As a magnetic field is applied, the free electrons inside start to move in a circle,
a process called cyclotron motion.
Microwaves are then passed through the gas at a frequency exactly in sync,
or in resonance with, the electrons’ motion.
This increases their kinetic energy, and knocks even more electrons off the gas,
creating an energetic ionized plasma.
The ions are then accelerated by electrically-charged grids,
and are expelled from the chamber to produce thrust.
Electron-cyclotron resonance is incredibly efficient and,
in contrast to cold gas propulsion, has a very large specific impulse.
Turning your source gas into energetic ions is a very effective way of
getting the most out of each molecule of propellant,
meaning your CubeSat can keep thrusting for a long time.
Using ions to create thrust isn’t a new idea.
NASA tested an ion engine way back in 1964, and between 1998 and 2001
the Deep Space 1 mission traveled nearly 264 million kilometers by ion propulsion alone.
But using ECR to generate ions can potentially increase mission length even further,
since there aren’t any electrodes to get damaged or worn away.
On the other hand, the absolute thrust from ion propulsion is pretty small,
so these CubeSats won’t be winning any drag races.
This makes them less suitable for missions in low-Earth orbit,
where fast maneuvers are needed to overcome atmospheric drag.
Plus, with gases, magnetic fields, microwaves, and charged grids to manage,
there’s a lot of interconnected parts.
Microwaves are particularly difficult to manage.
If the frequency isn’t exactly right, it won’t excite the electrons at all.
And even when conditions are perfect for resonance,
that will happen only in a small section of the gas chamber.
This part needs to be protected, so that the excited electrons don’t hit the chamber’s wall
and expend all their energy before creating ions from the rest of the gas.
Researchers are still working to keep the resonance contained,
and fit everything within the power and mass budget for a tiny satellite.
Because CubeSats themselves are so versatile, each mission will have specific propulsion needs.
For some, the simplicity of cold gas will do.
For others, the high mileage of ECR will be a better fit.
I am very happy to be a mini-propulsion system salesperson, just call me up.
I feel like I could do that job.
But with exciting developments on all fronts,
there will soon be a mini-propulsion system to suit any need.
And to suit your need for more good satellite content, did you know that SciShow has a podcast?
It’s true! It’s called SciShow Tangents and on it three friends and I
share our enthusiasm for science while trying really hard not to venture too far off topic.
If you haven’t listened to it yet,
I suggest you start with our third ever episode because it’s all about satellites.
And a little bit about Halloween.
You can check out the link in the description,
or search for SciShow Tangents wherever you listen to podcasts.
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